1. Field of the Invention
The present invention relates to an air-fuel ratio feedback control for an engine which executes an air-fuel ratio feedback control of air-fuel mixture based on air-fuel ratio detection results of air-fuel ratio sensors disposed at both upstream and downstream sides of a catalytic converter.
2. Related Art
A conventional air-fuel ratio control system for engines executes an air-fuel ratio feedback control of air-fuel mixture to be supplied to an engine, based primarily on the air-fuel detection result of an air-fuel ratio (A/F) sensor disposed at the upstream side of a catalytic converter. However, component parts of the system, such as air-fuel ratio sensors and fuel injection valves, have variations in the operation characteristics that occurs at the time of production and/or during use. This results in errors in the conventional air-fuel ratio feedback control.
It is therefore proposed to provide a dual sensor system which uses an air-fuel ratio sensor additionally at the downstream side of the catalytic converter, so that the air-fuel ratio feedback control is executed based also on the air-fuel ratio detection result of the downstream air-fuel ratio sensor.
In such a dual sensor system as disclosed in Japanese Patent No. 2518254, for instance, air-fuel ratio feedback control gain is set variably in accordance with a parameter such as an intake air amount, engine rotational speed or intake air pressure which represents an exhaust gas transport delay. As the exhaust gas transport delay causes a delay in the air-fuel ratio detection at the downstream side of the catalytic converter, it is disadvantageous to the air-fuel ratio feedback control. Therefore, in the above dual sensor system, speed of varying the feedback control gain is set larger as the exhaust gas transport delay becomes large. Although this dual sensor system reduces the feedback control error resulting from the exhaust gas transport delay, it is not sufficient to control the air-fuel ratio accurately.